In wireless telecommunication systems, the transmit signal typically occupies a well-defined range of the frequency spectrum and power emitted outside this frequency range is subject to maximum emission limits imposed by regulatory or other requirements. These requirements ensure communication equipment using different parts of the frequency spectrum do not excessively interfere with one another.
The 3GPP (3rd Generation Partnership Project) specification for E-UTRA (evolved UMTS Terrestrial Radio Access), better known as LTE (Long Term Evolution), currently lists 38 different frequency bands for use by the latest generation of cellular communication equipment. Many of these bands are located in close vicinity to each other or to frequency bands of incumbent technologies such as television broadcast.
In a number of cases, out-of-band emissions are restricted to very low levels just outside the allocated frequency bands. One example of particular interest relates to emissions from LTE band 13 (777 MHz to 787 MHz) into a US public safety band (769 MHz and 775 MHz). Another example is emissions from LTE band 1 (1920 MHz to 1980 MHz) into the PHS (Personal Handy-phone System) band (1884.5-1915.7 MHz) which is in use in Japan.
Emissions are also often restricted where two frequency bands are immediately adjacent to one another. For example, there is no gap between LTE bands 42 and 43 and therefore emissions from band 42 into band 43 and vice versa are restricted. Similar constraints apply to LTE bands 23 and 25 as well as bands 1 and 33. In the future, more frequency spectrum may be dedicated to cellular communication equipment and therefore coexistence requirements will increase.
Stringent emission limits that apply just a few Megahertz outside the desired transmit channel impose a number of challenging design constraints on the transmitter architecture.
FIG. 1 shows a simplified block diagram of a representative architecture of a state-of-the-art LTE transmitter (10). User data (11) is interleaved with control data (not shown) and modulated (12) using a technique called SC-FDMA (Single-Carrier Frequency Division Multiple Access) which yields a stream of time-domain data symbols (13). The signal comprising the symbols includes real and imaginary parts that are commonly referred to as I and Q (in-phase and quadrature components). The I and Q signal paths as shown propagate throughout the digital and analog parts of the architecture of the transmitter and are re-combined at the IQ modulator 20. Between the symbols 13 a cyclic prefix (14) is inserted to effectively create a guard time between the data symbols. At this point in the signal chain the frequency spectrum associated with the data stream is not very well confined to the desired bandwidth and must be shaped by digital filtering (15) to reject unwanted out-of-band emissions. Typically, the data stream is then up sampled (16) to a rate multiple times the native LTE symbol rate which by a process known as aliasing again produces unwanted out-of-band emissions as would be understood. These can be removed using the anti-aliasing filter (17).
The signal can then be converted from the digital into the analog domain using a DAC (18). The radio topology shown is known as a zero-IF architecture where the complex baseband signal is represented by two real-valued signal paths (in-phase and quadrature components, commonly referred to as I and Q) in the analog domain as would be understood. This type of architecture is common in low-cost transceiver designs based on CMOS technology.
Following the DAC, the signal is filtered again (19), mainly to remove DAC quantization noise at the duplex offset for Frequency Division Duplexing (FDD) radio bands. Then, the I and Q signal paths are jointly up converted onto an RF carrier in the IQ modulator block (20) as would be understood. The RF signal is then amplified (21) by an RF amplifier followed by a power amplifier and filtered again (22) before being transmitted from the antenna (23).
Emission limits as discussed previously set design constraints on a number of blocks shown in the architecture in FIG. 1. For example, the combination of pulse shaping (15), digital anti-aliasing filter (17) and analog reconstructing filter (19) must suppress out-of-band power at the critical frequency offsets adequately so they have negligible contribution to out-of-band noise after up-conversion (20) to RF and RF amplification (21).
Many linearization schemes or pre-distortion schemes have been proposed that aim to counteract non-ideal characteristics of the power amplifier with the aim of improving its linearity and power efficiency. In these schemes the correction signal is calculated to minimize spectral re-growth due to different frequency components in a wide-band signal mixing with each other. This improves the spectral characteristics of wide-band signals. However, the correction terms applied, themselves, have frequencies close to the desired frequencies and do not typically improve emissions at frequency offsets several multiples away from the desired frequency.